Add launchd logs to Init Logs checkup (or to their own checkup) on macOS

[RebeccaMahany]

In flares, we want to collect all launchd logs in the directory /var/log/com.apple.xpc.launchd/ (they will look like launchd.log, launchd.log.1, etc). In the event that launcher is being restarted repeatedly by launchd, these logs should help us understand why.

This is a little bit similar to gatherServiceManagerEventLogs and gatherServiceManagerEvents for Windows in the Service Report checkup, so it could make sense to do some refactoring and group all of these logs together in their own checkup -- or maybe the Init Logs checkup is a fine place for them to live.